The invention relates to a muffler for the exhaust line of a combustion engine, especially that of an airplane engine.
The muffling of gases streaming out of a combustion engine is a known problem. It is usually dealt with by means of exhaust mufflers such as are generally known for powered vehicles, namely a container which is built into the exhaust line and which is divided by means of various transverse walls, called baffle plates. The purpose is to catch the gases entering the line in a pulsed manner as an input and to pass them on in a uniform stream as an output thereby eliminating popping noises. As is known, however, this can be achieved only by a drop in performance of the combustion engine which results from the back-pressure of the muffler. A repeated change in the flow direction of the gases as they pass around the baffle plates leads to a substantial drop in the efficiency.
It has therefore already been attempted to solve the problem by inserting in the exhaust line a simple perforated inner pipe which is surrounded by an outer, casing pipe. The intervening space was filled with long filament glass wool. Glass wool, however, is not very resistant to heat, and therefore melts rapidly in use to a small portion of its original volume. This damage occurs after only a short time, especially for combustion engines of small planes, since such planes are mostly still equipped with piston engines for which the temperature of the exhaust gas is higher (up to 500 degrees Celsius) than that for powered vehicle motors.
Although mufflers having the mentioned baffle plates provide long service, the muffling is nevertheless only a partial aspect. It is equally essential that this muffling also be obtained with the gases having the least possible back-pressure effect in passing through. It is possible to achieve this with very long pipes, but usually there is not enough space. There remains the fact that the demand for unrestricted passage of the exhaust gases is generally in conflict with the demand for the greatest possible muffling. Finally, there must also be considered the problem of joining together the individual parts, as well as that of maintenance. It is known that the mufflers which are ordinarily of welded construction show cracks after a time which necessitate premature replacement. The formation of these cracks takes place primarily at very high exhaust gas temperatures, and therefore is primarily found in mufflers for airplane motors. The welded seams are there stressed beyond their rating, due to the temperature differentials and the resulting differential expansion of the gas-carrying inner pipe and the other muffler parts.
One purpose of the invention is therefore to provide a muffler which combines a long-lasting muffling action for the exhaust gases generated by the motor with a practically unimpeded passage of the exhaust gases, for a minimum performance drop, so that either a smaller motor can be used for a given performance rating or a given motor can offer a higher performance.
The muffler should be suitably constructed at the connecting points between the hot and the comparatively cold parts without welding, in order to permit a differential expansion of these parts without thermal overstressing. There is also desirable a ready disassembly of the muffler for checking and maintenance.